1. Field of the Invention
The present invention relates to a method for producing a titanium strip having a small proof strength anisotropy and improved ductility, by a strip rolling method.
The term "proof strength anisotropy" denotes the ratio of a proof strength in a rolling direction (L direction) to that in a direction (T direction) perpendicular to the L direction.
2. Description of the Related Art
The production of pure titanium is usually carried out by the steps of hot rolling, annealing, pickling, cold rolling, and final annealing.
However, the usual hot rolled strips or sheets and cold rolled and annealed strips or sheets contain a remarkable proof strength anisotropy. Namely, an L direction value .sigma..sub.y L of a yield strength or a 0.2% proof strength (if a yield is not generated) is smallest and a T direction value is largest, whereby the proof strength anisotropy, i.e., the ratio .sigma..sub.y T/.sigma..sub.y L, is about 1.3. Therefore, the rolled pure titanium has an overhang, and this leads to shape defects during fabrication, such as deep drawing, remarkable earing generation or a press cracking.
To solve these problems, the conventional methods of cross rolling and slight rolling process after annealing, etc., are widely used.
However, the cross rolling process can not be used for unidirectional rolling process, such as for the strip rolling.
Further, in the slight rolling process, the effects which solve the above-mentioned problem are lost by a full annealing.
Japanese Unexamined Patent Publication (Kokai) No. 60-194052 discloses a method for producing an titanium strip wherein a titanium hot rolled strip having an oxygen content of 0.25% by weight and an Fe content of 0.20% by weight is cold rolled by an undirectional rolling, and annealed and this cold rolling and annealing are repeated, whereby the proof strength anisotropy of the obtained titanium strip can be kept lower than 1.15.
Namely, in the above process, the proof strength anisotropy .sigma..sub.0.2 (T)/.sigma..sub.0.2 (L) is within 1.07 to 1.15.
However, the properties of the strength and the ductility of the obtained titanium strip are the same as a high strength and a low ductility type strip and thus can be used as a high strength member, but cannot be used as a fabrication material due to the poor ductility thereof.